Data center networks have become increasingly complex in order to support progressively more complicated switching and pathing topologies. For example, a Layer 2 network (i.e., an Ethernet network) may be configured to support multipathing between nodes in a data center network cloud. One emerging approach for allowing Layer 2 multipathing is TRILL, short for Transparent Interconnection of Lots of Links. A standards-compliant TRILL cloud configured using a standards-compliant IS-IS protocol (intermediate switch to intermediate switch) supports multipathing between nodes of the network cloud. At the juncture of such a network cloud with a Classical Ethernet (CE) network, the multipathing benefits should ideally be extended to the CE switches. However, when a CE switch is dual-homed (or multi-homed) to two (or more) nodes in the TRILL cloud, in a pure spanning tree network, all but one of the links to the TRILL cloud may be blocked, leading to inefficient link utilization. That is, even though a CE switch may include links to multiple switches in the TRILL cloud, spanning tree may block one (or more) of the links from being used to prevent loops in a Layer 2 network traffic.
Technologies such as virtual Port Channel (vPC) alleviate some of these inefficiencies in a CE cloud and allow dual-homed hosts connected to multiple aggregation switches to use all the links by creating a single logical port channel that includes a physical link to each switching device within the cloud. Thus, with vPC, multipathing support may be extended to a CE cloud. Traffic from the CE device, originating from the same MAC, can be load balanced and land on different switches in the TRILL cloud and forwarded. That is, while the CE switch treats multiple, physical links as a single port channel (with respect to spanning tree), the CE switch may load balance traffic into the cloud over the underlying physical links to different TRILL switches. As a result, connecting a CE device through a virtual port-channel to multiple TRILL switches creates important challenges for other switches within the network cloud. For example, with optimized MAC-learning, nodes behind the virtual port channel inside the TRILL cloud could see the MAC address flip-flop as being learned off one TRILL switch and then off the other as traffic is load-balanced across the two.